Drive circuit and semiconductor apparatus for a high output motor

ABSTRACT

According to embodiments, a drive circuit includes a control portion configured to, when a duty ratio is set to a first predetermined value, and a load apparatus is in an overload state, set the duty ratio to a second predetermined value smaller than the first predetermined value during a predetermined time period.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromU.S. Provisional Application No. 62/297,231, filed on Feb. 19, 2016; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a drive circuit and asemiconductor apparatus.

BACKGROUND

Conventionally, there is a PWM control technique, for example, for amotor drive circuit. In the PWM control technique, a command voltagesignal for commanding a duty ratio is inputted; an output PWM signalwith a predetermined frequency is generated based on the command voltagesignal; and a load apparatus is controlled by the output PWM signal.

In a drive circuit to which the PWM control technique is applied,especially in a drive circuit for driving a motor, when a coil currentflowing through a coil of the motor exceeds a predetermined threshold,an internal PWM signal G is lowered from an H level to an L level togenerate an output PWM signal so that an overload current does not flowthrough the motor.

However, in a conventional drive circuit, if the duty ratio is set to100% in order to maximize output of a motor, the H level of the internalPWM signal is maintained until the coil current of the motor exceeds thepredetermined threshold, and a frequency may be lower than thepredetermined frequency.

When the frequency of the internal PWM signal becomes lower than thepredetermined frequency, a frequency of the output PWM signal alsobecomes lower than the predetermined frequency. The motor driven by theoutput PWM signal having a lowered frequency may cause, for example,noise by an electromagnetic sound, or vibration, noise and the like byincrease in a current ripple.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of main portions of adrive circuit according to a first embodiment;

FIG. 2 is a block diagram showing a configuration of a control portionof the drive circuit according to the first embodiment;

FIG. 3 is an explanatory diagram illustrating a duty ratio setting flowof the drive circuit according to the first embodiment;

FIG. 4A is a timing chart showing an example of a command signalinputted to the drive circuit according to the first embodiment;

FIG. 4B is a timing chart illustrating an example of the duty ratio setby a judgment portion of the drive circuit according to the firstembodiment;

FIG. 4C is a timing chart illustrating an example of a limit signaloutputted by a limit signal generating portion of the drive circuitaccording to the first embodiment;

FIG. 4D is a timing chart illustrating an example of an internal PWMsignal outputted by the control portion of the drive circuit accordingto the first embodiment;

FIG. 5A is a timing chart illustrating an example of the duty ratio setby the judgment portion of the drive circuit according to the firstembodiment;

FIG. 5B is a timing chart illustrating an example of the limit signaloutputted by the limit signal generating portion of the drive circuitaccording to the first embodiment;

FIG. 5C is a timing chart illustrating an example of the internal PWMsignal outputted by the control portion of the drive circuit accordingto the first embodiment;

FIG. 6 is a block diagram showing a configuration of main portions of adrive circuit according to a second embodiment;

FIG. 7A is a timing chart illustrating an example of a limit signaloutputted by a limit signal generating portion of the drive circuitaccording to the second embodiment;

FIG. 7B is a timing chart illustrating an example of a conversion signaloutputted by a limit signal converting circuit of the drive circuitaccording to the second embodiment;

FIG. 7C is a timing chart illustrating an example of a subtractionsignal and a triangular wave signal outputted by a subtraction circuitand a triangular wave generating portion of the drive circuit accordingto the second embodiment;

FIG. 7D is a timing chart illustrating an example of an internal PWMsignal outputted by a control portion of the drive circuit according tothe second embodiment; and

FIG. 7E is a timing chart illustrating an example of a duty ratio of thedrive circuit according to the second embodiment.

DETAILED DESCRIPTION

Drive circuits of embodiments have a control portion configured to, whena duty ratio is set to a first predetermined value, and a load apparatusis in an overload state, set the duty ratio to a second predeterminedvalue smaller than the first predetermined value during a predeterminedtime period.

(First Embodiment)

A first embodiment will be described below with reference to drawings.

(Configuration)

FIG. 1 is a block diagram showing a configuration of main portions of adrive circuit 1 according to a first embodiment. FIG. 2 is a blockdiagram showing a configuration of a control portion 11 of the drivecircuit 1 according to the first embodiment.

As shown in FIG. 1, the drive circuit 1 is configured with asemiconductor apparatus and has the control portion 11, a drive signaloutputting portion 21, a motor driving portion 31 which is a loadapparatus driving portion, a comparator 41 and a limit signal generatingportion 51. A motor M, which is a load apparatus, is connected to themotor driving portion 31.

The control portion 11 is a circuit configured to set a duty ratio andgenerate an internal PWM signal G corresponding to the duty ratio. Thecontrol portion 11 is connected to the drive signal outputting portion21. The control portion 11 is capable of outputting the internal PWMsignal G to the drive signal outputting portion 21. The control portion11 has an AD conversion portion 12, a judgment portion 13 and a dutygenerating portion 14.

A command signal is inputted to the control portion 11 so that a dutyratio corresponding to a voltage value can be set. The command signalmay be a voltage signal, for example. A voltage width of the commandsignal, for example, from 0 V to 5 V is set in advance. For example,when the voltage width of the command signal is set from 0 V to 5 V, acommand value for commanding a duty ratio for a command signal of 0 V is0%, and a command value for commanding a duty ratio for a command signalof 5 V is 100%. Note that the command signal may be a PWM periodicsignal. For example, when an input duty ratio defined by the PWMperiodic signal is 0%, a command value is 0%, and when the input dutyratio is 100%, the command value is 100%.

The AD conversion portion 12 is a circuit configured to convert aninputted command signal to a digital signal. The AD conversion portion12 is connected to the duty generating portion 14. The AD conversionportion 12 converts an inputted command signal to a digital signal andoutputs the digital signal to the duty generating portion 14.

The judgment portion 13 is a circuit configured to perform judgmentabout duty ratio based on a duty ratio commanded by a command signal anda limit signal LS inputted from the limit signal generating portion 51to set a duty ratio. The judgment portion 13 has a command value settingportion 13 a, an overload detecting portion 13 b and a timer 13 c.

The command value setting portion 13 a acquires a digitized commandsignal from the duty generating portion 14, reads a command value fromthe command signal and sets a duty ratio corresponding to the commandvalue. That is, the command value setting portion 13 a sets a duty ratiobased on a command signal.

The overload detecting portion 13 b detects whether the motor M, whichis a load apparatus, is in an overload state. More specifically, theoverload detecting portion 13 b detects whether an overcurrent flowsthrough a coil of the motor M based on the limit signal LS inputted fromthe limit signal generating portion 51 to be described later.

The timer 13 c starts operation in response to an operation instructionof the judgment portion 13 and stops the operation after elapse of apredetermined time period T. A function of the timer 13 c may berealized by counting pulses of the internal PWM signal G a predeterminednumber of times or may be realized by an internal clock not shown. Forexample, when a frequency of the internal PWM signal G is 20 KHz, thepredetermined number of times is 2000. The judgment portion 13 can judgewhether the predetermined time period T has elapsed or not by the timer13 c.

That is, when the duty ratio is set to a first predetermined value, anda load apparatus is in an overload state, the control portion 11 setsthe duty ratio to a second predetermined value smaller than the firstpredetermined value during the predetermined time period T.

More specifically, when the duty ratio is set to the first predeterminedvalue, and the limit signal LS is inputted from the limit signalgenerating portion 51, the control portion 11 sets the duty ratio to thesecond predetermined value and outputs the internal PWM signal G basedon the duty ratio.

More specifically, when the duty ratio with the first predeterminedvalue is set by the command value setting portion 13 a, and the limitsignal LS is detected by the overload detecting portion 13 b, and thecontrol portion 11 judges that an overcurrent flows through the coil ofthe motor M, the control portion 11 sets the duty ratio to the secondpredetermined value during the predetermined time period T by the timer13 c and outputs the set duty ratio to the duty generating portion 14.

The duty generating portion 14 is a circuit configured to generate aninternal PWM signal G according to the duty ratio judged and set by thejudgment portion 13. The duty generating portion 14 is connected to thedrive signal outputting portion 21. The duty generating portion 14generates an internal PWM signal G and outputs the internal PWM signal Gto the drive signal outputting portion 21.

The frequency of the internal PWM signal G is set, for example, to afrequency higher than 20 KHz which is a human's audible range.

The drive signal outputting portion 21 is a circuit configured to outputan output PWM signal to the motor driving portion 31 as a drive signalfor driving the motor driving portion 31. The drive signal outputtingportion 21 is connected to the control portion 11, the limit signalgenerating portion 51 and the motor driving portion 31. When a limitsignal LS is inputted from the limit signal generating portion 51, thedrive signal outputting portion 21 lowers the duty ratio of the internalPWM signal G to generate an output PWM signal and outputs the output PWMsignal to the motor driving portion 31 so that an overcurrent does notflow through the coil of the motor M.

The motor driving portion 31 is a circuit configured to drive the motorM. The motor driving portion 31 is configured, for example, with athree-phase inverter circuit having an FET bridge with six FETs and thelike. Each FET is provided with a reflux diode not shown. The motordriving portion 31 is connected to the motor M, the comparator 41 and aresistor R. The motor driving portion 31 drives gates of the FETs basedon the output PWM signal inputted from the drive signal outputtingportion 21, generates a three-phase coil current for driving the motor Mand outputs the three-phase coil current to the motor M. The coilcurrent outputted to the motor M is inputted to the motor drivingportion 31 again via the coil in the motor M, which is not shown. Thecoil current inputted from the motor M is outputted to the comparator 41and the resistor R. Note that the configuration of the motor drivingportion 31 is not limited to the three-phase inverter circuit but may beother circuits, for example, an H bridge circuit with four FETs.

The motor M is configured, for example, with a brushless motor. Themotor M is configured having a stator wound by a three-phase coilconstituted by a U phase, a V phase and a W phase, which is not shown,and a rotor not shown.

The resistor R is arranged so as to be capable of measuring a coilcurrent value of the motor M. One end of the resistor R is connected tothe motor driving portion 31, and the other end is grounded. Voltage ata connection point K between the resistor R and the motor drivingportion 31 is calculated by multiplying the coil current value of themotor M and a resistance value of the resistor R together. Since theresistance value of the resistor R is set in advance, the coil currentvalue of the motor M can be calculated by the voltage at the connectionpoint K.

The comparator 41 is a circuit configured to detect whether the coilcurrent value of the motor M exceeds a predetermined threshold or not. Apositive side input terminal of the comparator 41 is connected to themotor driving portion 31. A negative side input terminal of thecomparator 41 is connected to a reference power source V. An outputterminal of the comparator 41 is connected to the limit signalgenerating portion 51. When the coil current value is lower than thepredetermined threshold, the voltage at the connection point K is lowerthan reference voltage inputted from the reference power source V, andthe comparator 41 outputs an L-level comparator signal to the limitsignal generating portion 51. When the coil current value is higher thanthe predetermined threshold, the voltage at the connection point K ishigher than the reference voltage, and the comparator 41 outputs anH-level comparator signal to the limit signal generating portion 51.

The reference voltage of the reference power source V is set in advanceso as to be lower than the voltage at the connection point K when thecoil current value of the motor M exceeds the predetermined threshold.

The predetermined threshold is set in advance in accordance with a kind,performance, and the like of the motor M.

The limit signal generating portion 51 is a circuit configured togenerate a limit signal LS based on an output signal of the comparator41. The limit signal generating portion 51 is connected to the controlportion 11 and the drive signal outputting portion 21. When a comparatorsignal inputted from the comparator 41 is an L-level signal, the limitsignal generating portion 51 outputs an L-level signal. On the otherhand, when the comparator signal inputted from the comparator 41 is anH-level signal, the limit signal generating portion 51 generates a limitsignal LS, which is a pulse wave, and outputs the limit signal LS to thecontrol portion 11 and the drive signal outputting portion 21.

That is, the limit signal generating portion 51 is connected to themotor driving portion 31 configured to drive the load apparatus,acquires a parameter value of the load apparatus from the motor drivingportion 31, and, when the load apparatus is in an overload state and theparameter value outputted from the load apparatus exceeds apredetermined threshold, outputs a limit signal LS indicating that theload apparatus is in the overload state, to the control portion 11 andthe drive signal outputting portion 21. More specifically, the limitsignal generating portion 51 is connected to the motor driving portion31 configured to drive the motor M, acquires the coil current value ofthe motor M from the motor driving portion 31, and, when the coilcurrent value exceeds a predetermined threshold, outputs a limit signalLS to the control portion 11 and the drive signal outputting portion 21.

(Operation)

Next, a duty ratio setting flow of the drive circuit 1 of the firstembodiment will be described.

FIG. 3 is an explanatory diagram illustrating the duty ratio settingflow of the drive circuit 1 according to the first embodiment. FIG. 4Ais a timing chart showing an example of the command signal inputted tothe drive circuit 1 according to the first embodiment. FIG. 4B is atiming chart illustrating an example of the duty ratio set by thejudgment portion 13 of the drive circuit 1 according to the firstembodiment. FIG. 4C is a timing chart illustrating an example of thelimit signal LS outputted by the limit signal generating portion 51 ofthe drive circuit 1 according to the first embodiment. FIG. 4D is atiming chart illustrating an example of the internal PWM signal Goutputted by the control portion 11 of the drive circuit 1 according tothe first embodiment.

When a command signal is inputted to the control portion 11, the ADconversion portion 12 converts the command signal to a digital signaland outputs the digital signal to the duty generating portion 14. FIG.4A is an example of starting input of a command signal 5V setting andcommanding a duty ratio of 100% at time TS.

The judgment portion 13 reads a command value (step S1). At step S1, thejudgment portion 13 acquires a digitized command signal from the dutygenerating portion 14 to read the command value.

The judgment portion 13 sets a duty ratio corresponding to the commandvalue (step S2). In the example of FIG. 4B, the judgment portion 13 setsthe duty ratio to 100% at the time TS (D1 in FIG. 4B).

The judgment portion 13 judges whether or not a limit signal LS isdetected (step S3). At step S3, if the limit signal LS is detected, theprocess proceeds to step S4. On the other hand, if the limit signal LSis not detected, the process returns to step S1. In the example of FIG.4C, a limit signal LS is detected at a time TL, and the process proceedsto step S4.

The judgment portion 13 judges whether the duty ratio is set to thefirst predetermined value or not (step S4). At step S4, if the dutyratio is set to the first predetermined value, the process proceeds tostep S5. On the other hand, if the duty ratio is not set to the firstpredetermined value, the process returns to step S1. In the example ofFIG. 4B, the first predetermined value is 100%. Therefore, if the dutyratio is set to 100%, the process proceeds to step S5.

The judgment portion 13 sets the duty ratio to the second predeterminedvalue (step S5). In the example of step S5, the judgment portion 13 setsthe duty ratio to the second predetermined value. In the example of FIG.4B, the second predetermined value is 95%. Therefore, the judgmentportion 13 sets the duty ratio to 95% at the time TL (D2 in FIG. 4B). Inthe judgment portion 13, the timer 13 c starts operation if the timer 13c is not operating.

The judgment portion 13 judges whether the predetermined time period Tof the timer 13 c has elapsed or not (step S6). If the predeterminedtime period T has elapsed after the timer 13 c started operation, theprocess returns to step S1. On the other hand, if the predetermined timeperiod T has not elapsed, the process returns to step S5. In the exampleof FIG. 4B, the process returns to step S1 at time TR, and the judgmentportion 13 sets the duty ratio to 100% by the processes of step S1 andS2 (D3 in FIG. 4B).

The process from step S1 to step S6 constitutes the duty ratio settingflow.

As shown in the example of FIG. 4D, the internal PWM signal G generatedby the duty generating portion 14 is at the H level (duty ratio of 100%)from the time TS to the time TL. Then, during the predetermined timeperiod T from the time TL to the time TR, the level of the internal PWMsignal G repeats the H level during a period Ta and the L level during aperiod Tb every cycle T1 (duty ratio of 95%). Then, at the time TR, thelimit signal LS is not detected because the coil current value of themotor M becomes smaller than the predetermined threshold, and theinternal PWM signal G becomes an H-level signal (duty ratio of 100%).

FIGS. 4B to 4D are an operation example in which, when the predeterminedtime period T elapses after the duty ratio is set to the secondpredetermined value, the coil current value of the motor M becomessmaller than the predetermined threshold, and the duty ratio returns tothe value commanded by the command signal.

Next, description will be made on an example of operation performed whenthe coil current value of the motor M exceeds the predeterminedthreshold after the predetermined time period T has elapsed.

FIG. 5A is a timing chart illustrating an example of the duty ratio setby the judgment portion 13 of the drive circuit 1 according to the firstembodiment. FIG. 5B is a timing chart illustrating an example of thelimit signal LS outputted by the limit signal generating portion 51 ofthe drive circuit 1 according to the first embodiment. FIG. 5C is atiming chart illustrating an example of the internal PWM signal Goutputted by the control portion 11 of the drive circuit 1 according tothe first embodiment.

As shown in the example of FIG. 5A, the judgment portion 13 causes theduty ratio to return from the second predetermined value (D2 in FIG. 5A)to the command value commanded by the command signal at the time TR (D3in FIG. 5A).

If the coil current value of the motor M does not become smaller thanthe predetermined threshold even after the predetermined time period Telapses, the limit signal generating portion 51 continuously outputs alimit signal LS, which is a pulse wave, to the overload detectingportion 13 b (LSc in FIG. 5B) as shown in the example of FIG. 5B.

When the limit signal LS is detected at time TLc, the judgment portion13 sets the duty ratio to the second predetermined value during thepredetermined time period T (D4 in FIG. 5A).

Thereby, the control portion 11 outputs the internal PWM signal G withthe duty ratio of 95% during the predetermined time period T from thetime TLc as shown in the example of FIG. 5C.

According to the first embodiment, when the duty ratio is set to apredetermined value, and the coil current of the motor M is in anovercurrent state, the drive circuit 1 drives the motor M to providehigh output and causes the frequency of the internal PWM signal G to bestable so that noise and vibration can be suppressed.

(Second Embodiment)

(Configuration)

Though the control portion 11 is configured with a digital circuit inthe first embodiment, the control portion 11 may be configured with ananalog circuit.

FIG. 6 is a block diagram showing a configuration of main portions of adrive circuit 1 a according to a second embodiment. In description ofthe second embodiment, components similar to those of the firstembodiment will be given same reference numerals, and description of thecomponents will be omitted.

The drive circuit 1 a is configured having a limit signal convertingportion 51 a and a control portion 11 a.

The limit signal converting portion 51 a is a circuit configured toconvert a limit signal LS. For example, the limit signal convertingportion 51 a is configured having a charge and discharge circuit and thelike. The limit signal converting portion 51 a is connected to the limitsignal generating portion 51 and a subtraction circuit 12 a. The limitsignal converting portion 51 a outputs a conversion signal generated byconverting a limit signal LS inputted from the limit signal generatingportion 51, to the control portion 11 a.

The control portion 11 a has the subtraction circuit 12 a, a triangularwave generating portion 13 b and a comparator 14 a.

The subtraction circuit 12 a is a circuit configured to generate asubtraction signal WS based on a command signal and a conversion signaland output the subtraction signal WS to the comparator 14 a. Thesubtraction circuit 12 a is connected to a positive side input terminalof the comparator 14 a. When a command signal with a first predeterminedvalue is inputted by filtering, the subtraction circuit 12 a subtracts aconversion signal from a command signal and outputs a subtraction signalWS to the comparator 14 a.

The triangular wave generating portion 13 b is a circuit configured togenerate a triangular wave signal S. The triangular wave generatingportion 13 b is connected to a negative side input terminal of thecomparator 14 a. The triangular wave generating portion 13 b generates atriangular wave signal S and outputs the triangular wave signal S to thecomparator 14 a.

The comparator 14 a is a circuit configured to generate an internal PWMsignal Ga and outputs the internal PWM signal Ga to the drive signaloutputting portion 21. The comparator 14 a is connected to the drivesignal outputting portion 21. When the subtraction signal WS inputtedfrom the positive side input terminal is higher than the triangular wavesignal S inputted from the negative side input terminal, the comparator14 a outputs an internal PWM signal Ga to be of the H-level to the drivesignal outputting portion 21.

In the second embodiment, a second predetermined value is defined as afunction of the subtraction signal WS outputted from the subtractioncircuit 12 a and the triangular wave signal S outputted from thetriangular wave generating portion 13 b.

(Operation)

Next, operation of the drive circuit 1 a according to the secondembodiment will be described.

FIG. 7A is a timing chart illustrating an example of the limit signal LSoutputted by the limit signal generating portion 51 of the drive circuit1 a according to the second embodiment. FIG. 7B is a timing chartillustrating an example of the conversion signal outputted by the limitsignal converting portion 51 a of the drive circuit 1 a according to thesecond embodiment. FIG. 7C is a timing chart illustrating an example ofthe subtraction signal WS and the triangular wave signal S outputted bythe subtraction circuit 12 a and the triangular wave generating portion13 b of the drive circuit 1 a according to the second embodiment. FIG.7D is a timing chart illustrating an example of the internal PWM signalGa outputted by the control portion 11 of the drive circuit 1 aaccording to the second embodiment. FIG. 7E is a timing chartillustrating an example of the duty ratio of the drive circuit 1 aaccording to the second embodiment.

When a command signal setting and commanding a duty ratio of 100% isinputted at the time TS, the coil current of the motor M exceeds apredetermined threshold at the time TL, and the limit signal generatingportion 51 outputs a limit signal LS to the limit signal convertingportion 51 a (FIG. 7A).

When the limit signal LS is inputted, the limit signal convertingportion 51 a converts the limit signal LS and outputs a conversionsignal to the subtraction circuit 12 a (FIG. 7B).

When the conversion signal is inputted, the subtraction circuit 12 asubtracts the conversion signal from the command signal and outputs asubtraction signal WS (FIG. 7C, solid line) to the comparator 14 a.

The comparator 14 a generates an internal PWM signal Ga based on thesubtraction signal WS inputted from the subtraction circuit 12 a and thetriangular wave signal S (FIG. 7C, a long dashed short dashed line)inputted from the triangular wave generating portion 13 b and outputsthe internal PWM signal Ga to the drive signal outputting portion 21(FIG. 7D).

The internal PWM signal Ga is at the H level from the time TS to thetime TL. Then, during the predetermined time period T from the time TLto the time TR, the duty ratio gradually increases (A1 to A4 in FIG.7D). Then, at the time TR, the internal PWM signal Ga becomes an H-levelsignal.

As shown in FIG. 7E, the duty ratio of the internal PWM signal Ga is100% until the time TL, becomes the second predetermined value definedas a function of the subtraction signal WS and the triangular wavesignal S between the time TL and the time TR, and becomes 100% at thetime TR.

According to the second embodiment, when the duty ratio is set to apredetermined value, and the coil current of the motor M is in anovercurrent state, the drive circuit 1 a drives the motor M to providehigh output and causes the frequency of the internal PWM signal Ga to bestable so that noise and vibration can be suppressed.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel circuits and systemsdescribed herein may be embodied in a variety of other forms;furthermore, various omissions, substitutions and changes in the form ofthe circuits and systems described herein may be made without departingfrom the spirit of the inventions. The accompanying claims and theirequivalents are intended to cover such forms or modifications as wouldfall within the scope and spirit of the inventions.

What is claimed is:
 1. A drive circuit comprising: a limit signalgenerating portion connected to a motor driving portion and configuredto output a limit signal, when a motor driven by the motor drivingportion comes into an overload state, and thereby a coil current valueof a coil current flowing through a coil of the motor exceeds apredetermined threshold, the coil current value being inputted from themotor driving portion; and a control portion configured to, when a dutyratio is set to a first predetermined value, and the limit signal isinputted from the limit signal generating portion, set the duty ratio toa second predetermined value smaller than the first predetermined valueduring a predetermined time period, and output an internal PWM signalbased on the set duty ratio, wherein the control portion comprises anoverload detecting portion; and the overload detecting portion detectsthat an overcurrent flows through the coil of the motor based on thelimit signal.
 2. The drive circuit according to claim 1, comprising adrive signal outputting portion connected to the limit signal generatingportion, the control portion, and the motor driving portion; whereinwhen the limit signal is inputted from the limit signal generatingportion, the drive signal outputting portion lowers the duty ratio ofthe internal PWM signal inputted from the control portion to generate anoutput PWM signal, and outputs the output PWM signal to the motordriving portion.
 3. The drive circuit according to claim 1, wherein thecontrol portion comprises a command value setting portion; and thecommand value setting portion sets the duty ratio corresponding to acommand value based on a command signal.
 4. The drive circuit accordingto claim 1, wherein the control portion comprises a timer configured tostart operation in response to an operation instruction and stop theoperation after the predetermined time period elapses; and the controlportion judges whether the predetermined time period elapses or not bythe timer.
 5. The drive circuit according to claim 4, wherein when theduty ratio with the first predetermined value is set by the commandvalue setting portion, and the limit signal is detected by the overloaddetecting portion, and the control portion judges that an overcurrentflows through the coil of the motor, the control portion sets the dutyratio to the second predetermined value during the predetermined timeperiod by the timer.
 6. The drive circuit according to claim 1, whereinthe first predetermined value is 100%.
 7. The drive circuit according toclaim 1, comprising a limit signal converting portion connected to thelimit signal generating portion; wherein the limit signal convertingportion comprises a charge and discharge circuit and generates aconversion signal obtained by converting the limit signal inputted fromthe limit signal generating portion.
 8. The drive circuit according toclaim 7, wherein the control portion comprises a subtraction circuitconnected to the limit signal converting portion; and the subtractioncircuit generates a subtraction signal based on a command signal and theconversion signal.
 9. The drive circuit according to claim 8, whereinthe control portion comprises a comparator connected to the subtractioncircuit; and the comparator generates the internal PWM signal based onthe subtraction signal inputted from the subtraction circuit and atriangular wave signal.
 10. A semiconductor apparatus comprising: alimit signal generating portion connected to a motor driving portion andconfigured to output a limit signal, when a motor driven by the motordriving portion comes into an overload state, and thereby a coil currentvalue of a coil current flowing through a coil of the motor exceeds apredetermined threshold, the coil current value being inputted from themotor driving portion; and a control portion configured to, when a dutyratio is set to a first predetermined value, and the limit signal isinputted from the limit signal generating portion, set the duty ratio toa second predetermined value smaller than the first predetermined valueduring a predetermined time period, and output an internal PWM signalbased on the set duty ratio.